The present invention relates to an improvement to gas burners for heating a gas flowing in a duct.
The technical field is making xe2x80x9cin-streamxe2x80x9d burners which are placed directly inside a gas flow duct, which burners are disposed as a rail generally made up of individual blocks and extending transversely across the gas flow direction.
In the present description, the term xe2x80x9cburnerxe2x80x9d is used both to cover a rail made up of a plurality of individual blocks, and an individual block on its own.
The main applications of the invention are in heating turbine gas in co-generator installations. A xe2x80x9cpost-combustionxe2x80x9d burner serves to improve the overall efficiency of the installation, to modulate the production of steam as a function of requirements, and, as a by-product, also serves to maintain such production in the event of the gas turbine stopping. Under such circumstances, a fan sucks in ambient air and delivers the oxygen required by the burners instead of and as a replacement for the exhaust gas from the turbine.
Burners must be capable of operating with a flame that is stable under conditions that are very different, i.e. with turbine gas at a temperature of 300xc2x0 C. to 600xc2x0 C. having 11% to 15% oxygen, or with ambient air. An example of a burner of this type is described in European patent 0 313 469 published on Apr. 23, 1989 in the name of Mxc3xa9canique Gxc3xa9nxc3xa9rale Foyers Turbine.
French regulations concerning emissions from installations of this type have recently been extended by an Aug. 11, 1999 Order issued by the French Environment and Planning Ministry limiting emissions of nitrogen oxides and carbon oxides. For boilers used in post-combustion, the specified values are: a maximum of 200 milligrams per standard cubic meter (mg/Nm3) for nitrogen oxides, NOx, and 250 mg/Nm3 for carbon oxides, CO.
Burners of the kind described in the above-cited European patent enable that order to be complied with while operating with turbine gas. However, even though certain devices are described in that patent for reducing nitrogen oxide emissions, they cannot in most cases achieve the values required by the regulations when operating post-combustion in ambient air.
The problem posed is thus to be able to make an xe2x80x9cin-streamxe2x80x9d burner operate while complying with the above values set by regulations and simultaneously preserving good flame stability.
A solution to the problem posed is a burner of conventional type which can be placed in a duct to heat a gas flowing in the duct. The burner comprises a pipe suitable for being placed transversely across the flow direction of the gas. The pipe is fed with fuel gas and is pierced by at least two holes that are in alignment on a common generator line thereof. The holes optionally are fitted with injectors and enable jets of fuel gas to be ejected in the downstream direction. The pipe carries a flame stabilizer formed by two deflector-forming fins diverging on either side of the generator line and serving to deflect the flow of oxidizing gas so as to create a protected zone in which flame can develop downstream from the burner rail.
According to the invention, at least one of the holes is extended by a tube extending beyond the outside edges of the fins and pierced by at least one fuel gas ejection orifice at its distal end.
Such a tube injects a portion of the fuel gas, for example at a distance of at least 100 millimeters (mm) downstream from the other orifices not fitted with tubes, thereby enabling the injection of the gas to be staged and enabling the resulting combustion flames to be staged, thus reducing the percentage of nitrogen oxides, NOx, that is produced. The main flame obtained by this staged injection, i.e. the flame which is further from the deflector stabilizer, is thus more aerated. Combustion thus takes place with a greater excess of air and therefor at a lower temperature, thereby producing a great reduction in the formation of those nitrogen oxides that are essentially of thermal origin. The greater the staging, the greater the extent to which nitrogen oxides are reduced.
Such a method of reducing nitrogen oxides, NOx, by staging the gas is already taught in the above-cited patent application EP 0 313 439; however, the tubes are mentioned therein only as being optional and they are situated outside the blocks making up the burner, with the ends of the tubes being situated in approximately the same plane as the leading edges of the fins of the stabilizer, and outside it.
The major drawback of that disposition is that the quantity of carbon monoxide, CO, that is formed is well above the limit allowed by the above-specified Order. This formation is due to the excessive distance between the flame which develops in the shelter of the stabilizer fins and the injection of secondary gas having a portion that is highly diluted by air and which therefore does not burn or does not burn completely.
Amongst other things, the present invention enables that defect to be remedied. The principle whereby the NOx is reduced remains that of staging the gas, but, in this case, the staging takes place axially instead of radially. To avoid forming CO, it suffices to maintain a sufficient flow rate (less than 30% of the total flow rate of the fuel gas) via the conventional orifices without tubes so as to create a pilot flame which ensures that the main flame coming from the staged injection ignites and is stable.
In a preferred implementation for improving aeration of the staged main flame, the distal end of the tube has at least four different ejection orifices. The gas can then be ejected through these various orifices at different angles, instead of being ejected through a single orifice on the axis of the tube. In the embodiment described and shown in the accompanying figures, the ejection axes of the orifices are inclined in pairs, firstly by an angle xcex2 of 20xc2x0 to 50xc2x0 on either side of the plane defined by the generator line on which the holes are in alignment and the axis of the pipe, and secondly, by an angle xcex1 of 10xc2x0 to 30xc2x0 on either side of the plane orthogonal to the preceding plane and containing the hole extended by the tube. The angles xcex1 and xcex2 are important in determining the quality of the results. They must be determined as a function of the dimensions of the burner and of the hearth on which it is mounted.
In addition, in order to be able to further improve this staging effect by reducing the primary flow rate, it is possible to conserve the stability of the main or xe2x80x9csecondaryxe2x80x9d flame since it achieves fuel gas staging with only 10xc2x0 of the gas in the primary orifices thus not having a tube. For this purpose, in accordance with the invention, it is possible to introduce a portion of the oxidizing gas at the root of the secondary flame. In addition to increasing the stability of the flame during turbine gas operation, this technique also has the advantage of reducing nitrogen oxide NO contained in the gas by the so-called xe2x80x9creburningxe2x80x9d effect (where NO is transformed into N2 by chemical reduction coming from the CH+ radicals present at the root of the flame). It is also possible to add at least one obstacle placed facing at least one gas jet ejected by one of the holes that is not extended by a tube, thereby encouraging them to expand and enlarge the pilot flame, which is also referred to as the xe2x80x9cprimaryxe2x80x9d flame, so as to impart better efficiency thereto.
Various shapes can be given to the obstacle, such as the preferred cylindrical shape. However, other shapes can also be used to similar effect.
The other characteristics of burners or burner rails as described in above-cited patent EP 0 313 469 can likewise be combined with those of the present invention described above and below in order to obtain a burner that also ensures a practically constant aeration rate for the air-and-fuel mixture in spite of variations in the fuel flow rate and/or in the speed of the gas to be heated. Such additional characteristics are described below and shown in some of the figures.
The result are novel gas burners for heating a gas flowing in a duct. The novel gas burners include improvements to presently known burners since they solve the same problem, which is that of complying with the values set by regulations concerning nitrogen oxide NOx and carbon oxide CO emissions, while ensuring good flame stability and an aeration rate that is practically constant in the air-and-fuel mixture regardless of variations in the fuel flow rate and/or in the speed of the gas to be heated.